FSO, FSOL or FSOC systems offer several advantages, such as large bandwidth, when compared to state-of-the-art free space radio frequency systems. FSO communications systems constitute an excellent choice in many new projects of high-transmission short links, among other things due to the free electromagnetic spectrum for utilization and the complete immunity to radio frequency interference as compared to radio systems. On the other hand, FSO systems are much more sensitive to atmospheric conditions such as mist, fog and refractive turbulence. These conditions cause higher intensity fading in the received optical signals than in radio waves.
Misalignment between two communicating FSO transceivers caused by e.g. building sway, seismic activity, hail and birds landing on the FSO transceivers, is another problem with FSO systems. This problem is addressed in U.S. Pat. No. 7,120,363-B2, wherein a method for automatic alignment of two FSO transceivers is disclosed. The FSO transceivers comprise optical elements, for example, optical signal generators, optical signal detectors and lenses for sending and receiving signals from optical fibers connected and comprised in the FSO transceivers. The optical elements are secured in a gimbal or optical cage, through which adjustment of both the azimuth and elevation of the optical elements is possible with the help of linear motors and a control device.
However there are further problems with FSO systems, one of them being caused by temperature fluctuations in the environment of FSO devices such as FSO transceivers. The manufacture of a completely optical communications device based on the utilization of a lens and an optical fiber, such as an FSO transceiver is rather complicated, considering that in an optical fiber the core diameter typically is a few micrometers and should receive all the concentrated energy that a relatively large area lens has captured. The distance between the lens and the optical fiber is very critical and should ideally be kept constant in any condition of a link in order to assure sufficient power levels. This is difficult to achieve in practice since the FSO transceivers are heated or cooled as a result of the weather conditions prevailing where they are positioned, since the parts in the FSO transceiver are expanded and contracted so that said critical distance is altered.